never turn off CHRONOMETER

Good afternoon everyone,
We are developing an app for the payment of services for the time of use, that is, you pay per minute that you are in a room. The time begins / ends when the app comes into contact with the devices located at the entrance and exit doors. The quality of our app is that it serves both data and no data (or we want) on the mobile. Obviously the payment can only be made when you have data.

In the OFF-LINE mode, I wanted to know if there is any way to count the time, activate a stopwatch, or get the exact time despite the user changing the time, turn off the mobile, erase the cache, etc .. that is, since it comes in contact with the input device (you have to press a button to start) until it comes into contact with the output device (same procedure). Being OFF-LINE can not send the information of when it comes in contact with the devices to the system.

I've seen that using the iOS timer, it does not matter if you close the app or turn off the mobile, it still works.
Thanks in advance,